buildroot

Revert "opencv: mark as not available on Xtensa"

Now that xtensa toolchain is able to compile huge functions this
workaround is no longer needed.

This reverts commit d21932508e69d4ab849cba9958f953ae9342cb8c.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>